Error with dataset value "-0" in calculated question type

The processing of "-0" as a dataset value is incorrect. I'm not sure the circumstances that create "-0", but moodle must be creating it as we have lots on our site. It's also possible to enter "-0" manually as a dataset value.

The bug is in evaluating a formula with the "-0". In qtype_calculate - >substitute_variables(), it won't use parentheses around "-0" since "-0" >= 0. For example. -

becomes in --0. This results in a PHP error being printed.

For format "Significant figures", this actually results in an infinite loop since there is no check that $answer is numeric before executing:

qtype_calculated_calculate_answer():

        $p10 = 0;

        while ($answer < 1) {

            --$p10;

            $answer *= 10;

        }

Suggested fixes:

1. In qtype_calculate->substitute_variables(), use parens for $val <= 0
2. In qtype_calculate->substitute_variables_and_eval(), use a different variable than $str in the eval(). Otherwise, if the eval() fails, the function will return the original formula. This triggers the infinite loop above.
3. Check for and reject "--" (and "++") in qtype_calculated_find_formula_errors().